U.S. patent application number 14/344809 was filed with the patent office on 2015-10-22 for carbon nanotube films processed from strong acid solutions and methods for production thereof.
This patent application is currently assigned to William Marsh Rice University. The applicant listed for this patent is Natnael Behabtu, Wing Kui Anson Ma, Mainak Majumder, Francesca Mirri, Jaewook Nam, Matteo Pasquali, Tien Yi Theresa Hsu Whiting. Invention is credited to Natnael Behabtu, Wing Kui Anson Ma, Mainak Majumder, Francesca Mirri, Jaewook Nam, Matteo Pasquali, Tien Yi Theresa Hsu Whiting.
Application Number | 20150298164 14/344809 |
Document ID | / |
Family ID | 47883738 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150298164 |
Kind Code |
A1 |
Pasquali; Matteo ; et
al. |
October 22, 2015 |
CARBON NANOTUBE FILMS PROCESSED FROM STRONG ACID SOLUTIONS AND
METHODS FOR PRODUCTION THEREOF
Abstract
In some embodiments, the present disclosure provides methods for
fabricating carbon nanotube films. Such methods generally comprise:
(i) suspending carbon nanotubes in a superacid (e.g. chloro
sulfonic acid) to form a dispersed carbon nanotube-superacid
solution, wherein the carbon nanotubes have substantially exposed
sidewalls in the carbon nanotube-superacid solution; (ii) applying
the dispersed carbon nanotube-superacid solution onto a surface to
form a carbon nanotube film; and (iii) removing the superacid.
Desirably, such methods occur without the utilization of carbon
nanotube wrapping molecules or sonication. Further embodiments of
the present disclosure pertain to carbon nanotube films that are
fabricated in accordance with the methods of the present
disclosure. Such carbon nanotube films comprise a plurality of
carbon nanotubes that are dispersed and individualized. Additional
embodiments of the present disclosure pertain to macroscopic
objects comprising the carbon nanotube films made in accordance
with the methods of the present disclosure described supra.
Inventors: |
Pasquali; Matteo; (Houston,
TX) ; Ma; Wing Kui Anson; (Storrs, CT) ;
Behabtu; Natnael; (Houston, TX) ; Majumder;
Mainak; (Wheeler's Hill, AU) ; Nam; Jaewook;
(Houston, TX) ; Mirri; Francesca; (Houston,
TX) ; Whiting; Tien Yi Theresa Hsu; (Doylestown,
OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pasquali; Matteo
Ma; Wing Kui Anson
Behabtu; Natnael
Majumder; Mainak
Nam; Jaewook
Mirri; Francesca
Whiting; Tien Yi Theresa Hsu |
Houston
Storrs
Houston
Wheeler's Hill
Houston
Houston
Doylestown |
TX
CT
TX
TX
TX
OH |
US
US
US
AU
US
US
US |
|
|
Assignee: |
William Marsh Rice
University
Houston
TX
|
Family ID: |
47883738 |
Appl. No.: |
14/344809 |
Filed: |
September 13, 2012 |
PCT Filed: |
September 13, 2012 |
PCT NO: |
PCT/US12/55183 |
371 Date: |
May 15, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61533888 |
Sep 13, 2011 |
|
|
|
Current U.S.
Class: |
428/161 ;
423/447.2; 427/384; 427/430.1; 427/557; 428/174; 428/220; 428/221;
428/408; 977/750; 977/892; 977/952 |
Current CPC
Class: |
C01B 32/00 20170801;
B05D 3/06 20130101; C01B 2202/22 20130101; B05D 1/305 20130101;
B82Y 30/00 20130101; B82Y 40/00 20130101; Y10S 977/75 20130101;
B05D 1/02 20130101; B05D 3/0272 20130101; C01B 32/174 20170801;
G02F 1/13338 20130101; B05D 1/18 20130101; B82Y 20/00 20130101;
C01B 2202/34 20130101; Y10S 977/952 20130101; Y10S 977/892
20130101 |
International
Class: |
B05D 1/18 20060101
B05D001/18; B05D 1/02 20060101 B05D001/02; C01B 31/00 20060101
C01B031/00; B05D 3/06 20060101 B05D003/06; B05D 3/02 20060101
B05D003/02; C01B 31/02 20060101 C01B031/02; G02F 1/1333 20060101
G02F001/1333; B05D 1/30 20060101 B05D001/30 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under Grant
Nos. FA9550-09-1-0590; FA9550-06-1-0207; FA8650-07-2-5061; and
FA8650-05-D-5807, all awarded by the U.S. Department of Defense.
The government has certain rights in the invention.
Claims
1. A method for fabricating carbon nanotube films comprising the
steps of: (i) suspending carbon nanotubes in a superacid to form a
dispersed carbon nanotube-superacid solution, wherein the carbon
nanotubes have substantially exposed sidewalls in the carbon
nanotube-superacid solution; (ii) applying the dispersed carbon
nanotube-superacid solution onto a surface to form a carbon
nanotube film on the surface; and (iii) removing the superacid.
2. The method of claim 1, wherein the carbon nanotubes are selected
from the group consisting of single-wall carbon nanotubes,
double-wall carbon nanotubes, multi-wall carbon nanotubes, small
diameter carbon nanotubes, ultra-short carbon nanotubes, and
combinations thereof.
3. The method of claim 1, wherein the superacid is selected from
the group consisting of fuming sulfuric acid, chlorosulfonic acid,
oleum, triflic acid, fluorosulfonic acid, trifluoromethanesulfonic
acid, perchloric acid, anhydrous hydrogen fluoride, and
combinations thereof.
4. The method of claim 1, wherein the superacid comprises
chlorosulfonic acid.
5. The method of claim 1, wherein the carbon-nanotube superacid
solution further comprises high performance materials selected from
the group consisting of graphene, fullerenes, boron nitride
nanotubes, hexagonal boron nitride, and combinations thereof.
6. The method of claim 1, wherein the suspending step comprises the
use of unfunctionalized carbon nanotubes.
7. The method of claim 1, wherein the formed carbon
nanotube-superacid solution is an isotropic solution, and wherein
the isotropic solution comprises individually dispersed carbon
nanotubes.
8. The method of claim 1, wherein the formed carbon
nanotube-superacid solution is a biphasic solution, and wherein the
biphasic solution comprises a mixture of both individually
dispersed carbon nanotubes and liquid crystals of aligned carbon
nanotubes.
9. The method of claim 1, wherein the formed carbon
nanotube-superacid solution is in liquid crystalline form.
10. The method of claim 1, wherein the applying step is selected
from the group consisting of filtration, printing, casting,
coating, dip coating, die coating, rod coating, spray coating, slot
coating, gravure coating, slide coating, knife coating, air knife
coating, curtain coating, screen coating, and combinations
thereof.
11. The method of claim 1, wherein the applying step comprises dip
coating.
12. The method of claim 1, wherein the surface is selected from the
group consisting of aluminosilicates, silicates, silicon oxides,
zeolites, glass, quartz, polymers, and combinations thereof.
13. The method of claim 1, wherein the surface comprises a
polymer.
14. The method of claim 13, wherein the polymer is selected from
the group consisting of polyethylenes, polyethylene terephthalate,
polypropylenes, polystyrenes, polyethylene furanoate,
polycyclohexylenedimethylene terephthalate,
polytetrafluoroethylene, fluorinated ethylene propylene,
perfluoroalkoxy, polyamides, polyimides, epoxies, aramids,
polyacrylonitriles, polyvinyl alcohols, polybutadienes, poly
acrylic acids, poly lactic acids, poly methacrylic acids,
polymethyl methacrylates, polyurethanes, poly vinyl chlorides,
polydimethyloxanes, polycarbonates, and combinations thereof.
15. The method of claim 1, wherein the removing step is selected
from the group consisting of direct coagulation, addition of
polymers, direct evaporation, and combinations thereof.
16. The method of claim 1, wherein the removing step occurs by
direct coagulation, and wherein the direct coagulation comprises
dipping the carbon nanotube film in pure solvents or mixtures of
solvents.
17. The method of claim 16, wherein the solvent comprises
chloroform, isopropanol, ether, acetone, and combinations
thereof.
18. The method of claim 16, wherein the removing step further
comprises heating by oven drying, exposure to hot gas, microwaving,
or combinations thereof.
19. The method of claim 16, wherein the removing step further
comprises washing with diethyl ether followed by water.
20. The method of claim 1, wherein the carbon nanotube film has a
thickness of about 1 nm to about 100 nm.
21. The method of claim 1, wherein the carbon nanotube film has a
thickness of about 10 nm to about 20 nm.
22. The method of claim 1, wherein the carbon nanotube film has an
average transmittance of about 60% to about 100% at 550 nm.
23. The method of claim 1, wherein the carbon nanotube film has an
average sheet resistance of about 20 ohm/sq to about 1530
ohm/sq.
24. The method of claim 1, wherein the carbon nanotube film is used
as a coating for a touch screen.
25. The method of claim 1, wherein the carbon nanotube film
comprises individualized carbon nanotubes.
26. The method of claim 1, wherein the carbon nanotube film
comprises long carbon nanotubes, wherein the long carbon nanotubes
comprise lengths that range from about 5 .mu.m to about 20
.mu.m.
27. The method of claim 1, wherein the carbon nanotube film
comprises isotropically oriented carbon nanotubes.
28. The method of claim 1, wherein the carbon nanotube film
comprises bundles of aligned carbon nanotubes.
29. The method of claim 1, wherein the carbon nanotube film
comprises a mixture of isotropically oriented carbon nanotubes and
bundles of aligned carbon nanotubes.
30. The method of claim 29, wherein the carbon nanotube film has a
conductivity range from about 1.1.times.10.sup.5 S/m to about
3.1.times.10.sup.5 S/m.
31. The method of claim 29, wherein the carbon nanotube film has a
conductivity range from about 2.5.times.10.sup.5 S/m to about
5.5.times.10.sup.6 S/m.
32. The method of claim 29, wherein the liquid crystals of aligned
carbon nanotubes have an ellipsoidal shape.
33. The method of claim 29, wherein the liquid crystals of the
aligned carbon nanotubes are thread-like.
34. The method of claim 1, wherein the method occurs without the
utilization of carbon nanotube wrapping molecules.
35. The method of claim 1, wherein the method occurs without the
utilization of sonication.
36. The method of claim 1, further comprising a step of separating
the carbon nanotube film from the surface.
37. A carbon nanotube film comprising a plurality of carbon
nanotubes, wherein the carbon nanotubes are dispersed and
individualized.
38. The carbon nanotube film of claim 37, wherein the carbon
nanotube film is freestanding.
39. The carbon nanotube film of claim 37, wherein the carbon
nanotube film is immobilized onto a surface.
40. The carbon nanotube film of claim 39, wherein the surface is
selected from the group consisting of aluminosilicates, silicates,
silicon oxides, zeolites, glass, quartz, polymers, and combinations
thereof.
41. The method of claim 39, wherein the surface comprises a
polymer.
42. The method of claim 41, wherein the polymer is selected from
the group consisting of polyethylenes, polyethylene terephthalate,
polypropylenes, polystyrenes, polyethylene furanoate,
polycyclohexylenedimethylene terephthalate,
polytetrafluoroethylene, fluorinated ethylene propylene,
perfluoroalkoxy, polyamides, polyimides, epoxies, aramids,
polyacrylonitriles, polyvinyl alcohols, polybutadienes, poly
acrylic acids, poly lactic acids, poly methacrylic acids,
polymethyl methacrylates, polyurethanes, poly vinyl chlorides,
polydimethyloxanes, polycarbonates, and combinations thereof.
43. The carbon nanotube film of claim 39, wherein the surface is
patterned, grooved, or non-planar.
44. The carbon nanotube film of claim 37, wherein the carbon
nanotubes are selected from the group consisting of single-walled
carbon nanotubes, double-walled carbon nanotubes, multi-walled
carbon nanotubes, small diameter carbon nanotubes, ultra-short
carbon nanotubes, and combinations thereof.
45. The carbon nanotube film of claim 37, wherein the film further
comprises high performance materials selected from the group
consisting of graphene, fullerenes, boron nitride nanotubes,
hexagonal boron nitride, and combinations thereof.
46. The carbon nanotube film of claim 37, wherein the carbon
nanotube film comprises isotropically oriented carbon
nanotubes.
47. The carbon nanotube film of claim 37, wherein the carbon
nanotube film comprises bundles of aligned carbon nanotubes.
48. The carbon nanotube film of claim 37, wherein the carbon
nanotube film contains a mixture of isotropically oriented carbon
nanotubes and bundles of aligned carbon nanotubes.
49. The carbon nanotube film of claim 48, wherein the bundles of
aligned carbon nanotubes have an ellipsoidal shape.
50. The carbon nanotube film of claim 37, wherein the carbon
nanotubes comprise long carbon nanotubes, wherein the long carbon
nanotubes comprise lengths that range from about 5 .mu.m to about
20 .mu.m.
51. The carbon nanotube film of claim 37, wherein the carbon
nanotube film has a thickness of about 1 nm to about 100 nm.
52. The carbon nanotube film of claim 37, wherein the carbon
nanotube film has a thickness of about 10 nm to about 20 nm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/533,888, filed on Sep. 13, 2011, the entirety of
which is incorporated herein by reference.
BACKGROUND
[0003] A large number of applications have been envisioned for
carbon nanotubes. However, their limited solubility in ordinary
solvents creates great difficulty in processing them into
macroscopic functional materials (such as fibers, films, and
composites). The limited solubility of carbon nanotubes also
impedes the development of multifunctional meta materials requiring
precise spatial ordering.
[0004] Furthermore, current methods and systems of providing
immobilized carbon nanotubes suffer from various limitations and
are not suitable for large-scale fabrication of thin films. Such
limitations include inability to produce one single electronic type
carbon nanotubes in large quantities. Such limitations also include
processing steps that adversely affect the mechanical and
electrical performance of the films. Therefore, there is currently
a need to develop new scalable processing methods and systems for
producing carbon nanotube films with controlled morphology and
optimal performance.
BRIEF SUMMARY
[0005] In some embodiments, the present disclosure provides methods
for fabricating carbon nanotube films. Such methods generally
comprise: (i) suspending carbon nanotubes in a superacid (e.g.
chlorosulfonic acid) to form a dispersed carbon nanotube-superacid
solution, wherein the carbon nanotubes have substantially exposed
sidewalls in the superacid solution; (ii) applying the dispersed
carbon nanotube-superacid solution onto a surface to form a carbon
nanotube film; and (iii) removing the superacid. Desirably, such
methods occur without the utilization of carbon nanotube wrapping
molecules (e.g., surfactants, soluble silicon oxide molecules,
and/or oligonucleotides) or sonication. In additional embodiments,
the methods of the present disclosure may further comprise: washing
the carbon nanotube film with diethyl ether followed by washing
with water.
[0006] Further embodiments of the present disclosure pertain to
carbon nanotube films comprising a plurality of carbon nanotubes,
where the carbon nanotubes are dispersed and individualized. In
some embodiments, such carbon nanotube films are made in accordance
with the methods of the present disclosure and do not require the
utilization of carbon nanotube wrapping molecules (e.g.,
surfactants) or sonication. Accordingly, in some embodiments, the
immobilized carbon nanotubes in the carbon nanotube films are not
associated with any carbon nanotube wrapping molecules. In some
embodiments, the carbon nanotube films of the present disclosure
may be freestanding. In some embodiments, the carbon nanotube films
of the present disclosure may be immobilized onto a surface.
[0007] Additional embodiments of the present disclosure pertain to
macroscopic objects comprising the carbon nanotube films made in
accordance with the methods of the present disclosure described
supra. As set forth in more detail herein, the methods and systems
of the present disclosure provide numerous improvements in
fabricating carbon nanotube films with controlled morphology and
optimal performance on various surfaces. In addition, the methods
and systems of the present disclosure provide various macroscopic
applications, including use of the carbon nanotube films made by
the methods of the present disclosure as films for touch screen
applications.
BRIEF DESCRIPTION OF THE FIGURES
[0008] FIGS. 1A-1B show transmitted light micrographs of dried
films dip coated from 0.1% (FIG. 1A) and 0.2% (FIG. 1B) isotropic
solutions of single-wall carbon nanotubes (SWNTs) in chlorosulfonic
acid.
[0009] FIG. 1C shows a scanning electron microscopy (SEM) image of
the film shown in FIG. 1B.
[0010] FIGS. 1D-1F show transmitted light micrographs of dried
films dip coated from biphasic solutions: 0.5% (FIG. 1D), 0.7%
(FIG. 1E), and 1% (FIG. 1F), respectively.
[0011] FIG. 2A depicts a schematic illustration of a film formation
process consisting of three main steps: (1) Self-assembly: SWNTs
are dissolved in chlorosulfonic acid creating a biphasic solution
which contains SWNT liquid crystals and individually dispersed
(isotropic) SWNTs; (2) Directed assembly: Dip coating is then
applied to produce a wet film containing elongated and aligned
liquid crystals; and (3) Self-assembly: by removing the acid
solvent, the pre-aligned liquid crystals serve as the nucleating
sites for whisker-like crystallites.
[0012] FIG. 2B shows experimental setup for dip coating the
SWNT-chlorosulfonic acid solutions. Both the dip coating and acid
removal steps were carried out inside a glove bag constantly purged
with anhydrous Argon to avoid moisture contamination.
[0013] FIGS. 3A-3B show optical micrographs of the dip coating
solution containing 0.7% (by vol.) SWNTs in chlorosulfonic acid,
imaged using transmitted light (FIG. 3A) and transmitted light with
cross-polarizers (FIG. 3B). The solution sample was sealed between
a glass microscope slide and a cover slip using aluminum tape.
[0014] FIGS. 3C-3D show optical micrographs of the corresponding
dip coated films (after acid removal) imaged under transmitted
light (FIG. 3C) and transmitted light with cross-polarizers (FIG.
3D). The dip coating direction was horizontal, as indicated.
[0015] FIGS. 4A-4B show SEM image (FIG. 4A) and transmission
electron microscopy (TEM) image (FIG. 4B) of a dried SWNT film
produced from the 0.5% (by vol.) SWNT-chlorosulfonic acid
solution.
[0016] FIG. 4C shows a SWNT whisker with a diameter of 100 nm
protruding out of a random SWNT network.
[0017] FIGS. 4D-4F show high magnification image of a SWNT whisker
showing high degree of SWNT alignment (FIG. 4D), the electron
diffraction pattern of the same whisker (FIG. 4E), and central line
profile of a SWNT whisker, indicating triangular lattice packing
(FIG. 4F).
[0018] FIGS. 5A-5E show shear rate profile for different
concentrations of dip coating solutions calculated from the 2-D
flow analysis: 0.2% (FIG. 5A), 0.5% (FIG. 5B), 0.7% (FIG. 5C), and
1% SWNT in chlorosulfonic acid (FIG. 5D). The dip coating solutions
are assumed as a power-law fluid. The values of maximum shear rate
(close to the glass substrate), average shear rate, and residence
time are given for each dip coating solution. The streamline plot
showing the liquid entrainment area (Ae) is illustrated in FIG.
5E.
[0019] FIGS. 6A-6B show dry film thickness (hdry) of films dip
coated from different concentrations of SWNT solutions measured by
AFM and profilometer (FIG. 6A). The substrate withdrawal speed is 1
mm/s. For the profilometer measurements, the error bar of each data
point represents the standard deviation of nine measurements (three
films dip coated from the same solution and three measurements are
carried out at various locations on each film). For AFM
measurements, the error bar represents three measurements carried
out at different locations of the same film. Film thickness as a
function of substrate withdrawal speed for three different
concentrations (CSWNT) of SWNT in chlorosulfonic acid is shown in
FIG. 6B. The lines are power law fit.
[0020] FIGS. 7A-7E depicts schematics of a dip coating process:
homogeneous solution of CNTs in chlorosulfonic acid (FIG. 7A);
withdrawal step and formation of the film on the glass slide by
controlling the lifting speed (FIG. 7B); coagulation and washing
steps for the removal of CSA using a chloroform bath or a series of
baths of chloroform coagulation followed by diethyl ether and water
washes (FIG. 7C); dip coating set-up (FIG. 7D); and 90% transparent
thin film obtained by dip coating from double-wall carbon nanotube
(DWNT)-CSA solutions (FIG. 7E).
[0021] FIGS. 8A-8B show micrographs of carbon nanotube films.
Transmitted light micrographs of SWNT (top) and DWNT (bottom) films
fabricated from a 1000 ppm solution at different withdrawal speeds
are shown in FIG. 8A. The scale bar is equal to 50 .mu.m and the
red arrow represents the coating direction. Polarized light
micrographs of SWNT (top) and DWNT (bottom) films fabricated from
1000 ppm solution at different withdrawal speeds (1 mm/min to 3
mm/min) are shown in FIG. 8B. SWNT films show isotropic orientation
at low shear rate. Slightly ordered structures in the coating
direction can be seen at 3 mm/min coating speed. DWNT films show a
preferential orientation in the coating direction due to the liquid
crystalline domains in the 1000 ppm CSA solution. These domains are
stretched during dip coating and yield ordered CNT bundles in the
film (bright regions). The films were coagulated with chloroform.
The scale bar is equal to 50 .mu.m. The red and white arrows
represent the coating direction and the cross polars,
respectively.
[0022] FIGS. 9A-9B show SEM images (FIG. 9A) and transmission
electron microscopy (TEM) images (FIG. 9B) of the films transferred
onto TEM grids. Presence of an isotropic network of individual CNTs
and bundles are likely responsible for the isotropic electrical
properties of the films. The films were chloroform coagulated
before being detached from the glass slide with a water bath.
[0023] FIGS. 10A-10B shows transmittance at the wavelength of 550
nm with respect to the withdrawal speed at different fixed
concentrations of DWNT in CSA (FIG. 10A). Each value of
transmittance is an average of 3 films fabricated with the same
dip-coating parameters. Transmittance versus sheet resistance for
films obtained by DWNTs using different acid removal processes is
shown in FIG. 10B. Films produced with direct CSA evaporation and
simple chloroform coagulation showed lower sheet resistance values
most likely due to the presence of sulfuric acid (see XPS data in
FIG. 16). On the other hand, higher values of sheet resistance were
found for films coagulated with chloroform and washed with diethyl
ether and water due to the near complete removal of sulfuric
acid.
[0024] FIG. 11 shows transmittance versus sheet resistance for
films obtained by DWNTs and SWNTs using simple chloroform
coagulation. Films obtained by SWNTs showed higher sheet resistance
(from about 4 to 10 times higher) than DWNT films. The length of
the CNTs plays a fundamental role in the film conductivity as
demonstrated in previous literature. Each sheet resistance value
represents the average of at least 5 random points in the film.
[0025] FIGS. 12A-12B show transmittance versus sheet resistance for
films obtained by dip coating in the recent literature (FIG. 12A),
and transmittance versus sheet resistance of the best films
obtained in literature up to date for various coating methods (FIG.
12B). The values of sheet resistance reported represent the average
of at least 5 random points in the film and were obtained for DWNT
films with the three techniques used.
[0026] FIG. 13 shows micrographs of 1000 ppm solutions of DWNT
(left) and SWNT (right) in CSA under cross-polarized light in 1 mm
thick glass capillaries. The DWNT solution shows strong
birefringence, indicating the presence of liquid crystals. The SWNT
solution is isotropic, as it shows almost no birefringence. The
small bright dots may be small liquid crystalline domains formed by
the longest SWNTs in the polydisperse sample. The capillaries were
filled by capillary forces in a low-humidity glovebox and then
flame sealed to avoid the ingress of moisture.
[0027] FIG. 14 shows viscosity versus shear rate for DWNT solutions
at 1000, 2000 and 3000 ppm concentrations. The solutions shear thin
as power-law fluids with apparent viscosity .eta..sub.a=K{dot over
(.gamma.)}.sup.n-i, where {dot over (.gamma.)} is the shear rate
and K is the consistency index. The power law exponent n can be
obtained from the slope of the curves as shown in the figure. Data
were collected using a AR2000eX rheometer with a concentric
cylinder (Couette) geometry. The rheometer was enclosed in a
glovebox to protect the sample from moisture.
[0028] FIG. 15 shows logarithmic plot of -lnT (where T is the film
transmittance) versus lifting speed u for films obtained at
different concentrations and withdrawal speed. The transmittance at
each concentration and lifting speed was calculated by averaging
three different films made under the same dip coating conditions.
The slope of 2/3 represents the theoretical value for Newtonian
fluids. The slope of 0.43, 0.42, and 0.41 are the slopes obtained
from the Gutfinger and Tallmadge model for 3000, 2000, and 1000 ppm
solutions, respectively.
[0029] FIG. 16 shows XPS spectra for films made using methods (1),
(2) and (3). Sulfur peaks appear clearly in films coagulated with
chloroform (method (1)) and vacuum dried-diethyl ether washed films
(method (3)) (about 9.1 and 2.3 atomic % of sulfur present using
method (1) and (3), respectively). The presence of sulfur is due to
the incomplete sulfuric acid removal. On the other hand, water
washed films (method (2)) removed the presence of sulfur peaks
(<0.1 atomic % sulfur content). The species % content was
estimated after the subtraction of the signal given by the bare
glass slide (Si and Na peaks come from the glass support as well as
most of the oxygen that showed approximately a ratio of 2.6:1 with
Si based on a control experiment with a clean glass slide) for an
average of five random points along each film. Data were collected
with PHI quantera XPS.
[0030] FIG. 17 shows electrical stability in time for films made
using three different CSA removal techniques: simple chloroform
coagulation (1); chloroform coagulation followed by diethyl ether
and water wash (2); and vacuum oven drying and diethyl ether wash
(3). On the left, sheet resistance versus time is shown. On the
right, sheet resistance relative to the initial sheet resistance
(R.sub.s/R.sub.so) versus time is shown. Films obtained with method
(1) were unstable in time (increase of 66.8% after 9 days) compared
to method (2) and (3) that exhibited higher stability. In
particular, using method (3), 91.4% transmittance film showed an
increase in sheet resistance of 13.6%, while using method (2), 93%
transmittance film and 77.5% transmittance film showed an increase
of 0.9% and of 8.6%, respectively. Each point represents the
average of five random spots on the film. The error bars indicate
the standard deviations.
[0031] FIG. 18 shows the effect of current on sheet resistance
R.sub.s(t) (normalized by its initial value R.sub.so) for films
with .about.90% transmittance. A current of 1 mA was applied for
180 min through the electrodes of a four-point probe. The film
produced by chloroform coagulation followed by diethyl ether and
water wash vacuum (method (2)) showed no significant change. Films
produced using simple chloroform coagulation (method (1)) and
vacuum oven drying and diethyl ether wash (method (3)) showed a
continuous increase in sheet resistance that plateaued after
.about.30 min (.about.40% increase for film (1) and .about.20%
increase for film (3)). The current was removed at t=180 min, and
Rs was monitored in time (by imposing current for a few seconds
every .about.5-60 min). The sheet resistance recovered slowly and
attained its initial value overnight, demonstrating that this
phenomenon is reversible and is most likely caused by the
separation of H.sup.+ and SO.sub.4.sup.2- ions at the electrodes of
the four-point probe.
DETAILED DESCRIPTION
[0032] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only, and are not restrictive of the invention, as
claimed. In this application, the use of the singular includes the
plural, the word "a" or "an" means "at least one", and the use of
"or" means "and/or", unless specifically stated otherwise.
Furthermore, the use of the term "including", as well as other
forms, such as "includes" and "included", is not limiting. Also,
terms such as "element" or "component" encompass both elements or
components comprising one unit and elements or components that
comprise more than one unit unless specifically stated
otherwise.
[0033] The section headings used herein are for organizational
purposes only and are not to be construed as limiting the subject
matter described. All documents, or portions of documents, cited in
this application, including, but not limited to, patents, patent
applications, articles, books, and treatises, are hereby expressly
incorporated herein by reference in their entirety for any purpose.
In the event that one or more of the incorporated literature and
similar materials defines a term in a manner that contradicts the
definition of that term in this application, this application
controls.
[0034] The development of new materials where carbon nanotubes are
dispersed and individualized is desirable for the development of
large-scale functional materials with novel properties (such as
improved sensors). Since their discovery, carbon nanotubes (CNTs)
have received increasing attention due to their outstanding
mechanical, thermal and electrical properties. In particular,
research has focused on realizing in macroscopic objects the
properties of single CNT molecules. For instance, CNTs have been
formed into neat fibers as well as thin conductive films. In
particular, transparent CNT films could replace indium tin oxide
(ITO) because of their flexibility, resistance to flexural fatigue,
and ease of manufacturing compared to the brittle ITO films, which
must be fabricated by sputtering at low pressure or chemical vapor
deposition at high temperature. Moreover, CNT films may enable new
applications in flexible electronics, because of their ability to
bend repeatedly without cracking.
[0035] Transparent conductive CNT films have been fabricated using
a variety of processes that include dry and wet methods. The dry
fabrication route consists of drawing films directly from CNT
arrays. Wet methods consist of dispersing the CNTs in a liquid, and
then fabricating films from the liquid phase. Multiple approaches
have been used for wet method thin film assembly, including vacuum
filtration, drop casting, spin coating, rod coating, spray coating,
and dip coating.
[0036] Although the fluid phase approach is more conducive to
industrial and commercial production, just a few of the above
techniques (rod, spray, and dip coating) are suitable for scale up
and can be adapted to high-throughput coating processes such as
slot, knife, slide, and roll coating. Furthermore, most liquid
phase film fabrication methods rely on functionalization or the use
of surfactants and sonication to form CNT dispersions. However,
functionalization degrades the electrical properties of the CNTs,
disrupting the sp.sup.2 bonds and yielding low film conductivity.
Moreover, surfactant stabilization relies on sonication, which
shortens the CNTs. Sonication can also degrade film conductivity
because it raises the number of CNT-CNT junctions per unit area of
the film.
[0037] Furthermore, good surfactants adsorb strongly on CNTs, and
their removal from the film is difficult. In addition, surfactant
residues in the final film increase sheet resistance. Therefore, a
solvent able to effectively disperse CNTs without damaging the
ultimate properties of the films is needed.
[0038] Chlorosulfonic acid (CSA) is a viable solution, and it
circumvents the potentially detrimental effects of sonication,
functionalization and use of surfactants. CSA-CNT solutions have
already been used for SWNT film fabrication. However, these
techniques were not scalable or yielded poor film properties.
Hence, the assembly of CNTs into large-scale functional materials
involves addressing important challenges, including organization,
alignment and individualization.
[0039] Presently, overcoming the aforementioned challenges is one
of the main goals of carbon nanotechnology. Thus, the present
invention aims to address these challenges.
[0040] Accordingly, one aspect of the present disclosure that will
be disclosed in more detail herein provides methods for the
production of high-performance thin transparent conductive CNT
films from superacids (e.g., CSA solutions) by various methods
(e.g., dip-coating). This is followed by superacid removal through
a series of steps (e.g., coagulation or drying, followed by
washing) that stabilizes the films and preserves the film structure
after fabrication. This process is scalable. Furthermore, no
sonication is needed. Therefore, in some embodiments, such
processes can produce films consisting of .about.10 .mu.m long CNTs
with optimal electrical properties.
[0041] The methods of the present disclosure also have several
advantages over prior art methods: Firstly, the films are naturally
acid-doped and have a higher conductivity. Secondly, this method
requires no sonication, making it highly adaptable for long
single-wall carbon nanotubes (SWNTs). Thirdly, by modulating the
interaction between CNTs through the use of different acid mixtures
with varying acid strength, one can further tailor the film
structure and properties. For instance, in various embodiments,
film morphology and the optical and electrical properties of the
film can be controlled by the coating speed, superacid
concentration, and level of doping. The level of doping can also be
controlled by post-processing, such as washing and heating at high
temperatures.
[0042] In some embodiments, the present disclosure provides methods
of making thin CNT films (e.g., CNT films with thicknesses in tens
of nanometers) from isotropic solutions of CNTs. In some
embodiments, the present disclosure pertains to the creation of
whisker-like crystallite structures from biphasic solutions of
carbon nanotubes. In some embodiments, these whisker-like
structures include SWNTs that are highly aligned and closely packed
in a crystalline manner. In some embodiments, the SWNTs in the
whisker-like structures have a maximum width of about 100 nm, and a
typical length of about 10-20 .mu.m. In some embodiments, these
whiskers are macroscopically aligned in the dip coating direction
and are interspersed with an isotropic SWNT network. In various
embodiments, the thickness of the SWNT films can be controlled
through the dip coating solution formulation and the substrate
withdrawal speed. Notably, the film-formation method closely
resembles the processing of liquid crystalline polymers into films
with aligned crystalline structures, confirming further that, with
the right solvent, polymer processing techniques can be readily
adapted to CNT material processing
[0043] Another aspect of the present disclosure pertains to a
carbon nanotube film comprising a plurality of carbon nanotubes,
where the carbon nanotubes are dispersed and individualized. Yet,
another aspect of the present disclosure pertains to macroscopic
objects comprising the carbon nanotube film made in accordance with
the methods of the present disclosure described supra.
[0044] The present disclosure provides a scalable method, by which
carbon nanotubes may be processed into films with controlled
morphology and optimal performance. In various embodiments, the
methods of the present disclosure require no sonication or chemical
functionalization. Thus, the methods of the present disclosure can
allow for the preservation of carbon nanotube length and intrinsic
electrical properties.
[0045] The aforementioned embodiments will be discussed in more
detail herein. Various aspects of the methods and systems of the
present disclosure will also be discussed with more elaboration
herein as specific and non-limiting examples.
[0046] Carbon Nanotubes
[0047] The methods and systems of the present disclosure may
utilize various types of carbon nanotubes. By way of background,
carbon nanotubes are nanoscale carbon structures comprising rolled
up graphene sheets. For instance, SWNTs comprise a single such
graphene cylinder, while multi-walled carbon nanotubes (MWNTs) are
made of two or more concentric graphene layers. Since their initial
preparation in 1993, SWNTs have been studied extensively due to
their unique mechanical, optical, electronic, and other properties.
For example, the remarkable tensile strength of SWNTs has resulted
in their use in reinforced fibers and polymer nanocomposites.
[0048] In some embodiments of the present disclosure, carbon
nanotubes used in conjunction with the methods and systems of the
present disclosure may include, without limitation, single-walled
carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs),
multi-walled carbon nanotubes (MWNTs), small diameter carbon
nanotubes (i.e., carbon nanotubes with diameters equal or less than
about 3 nm), ultra-short carbon nanotubes, and combinations
thereof. In some embodiments, the carbon nanotubes used in
conjunction with the methods and systems of the present disclosure
may include pristine carbon nanotubes, such as unmodified carbon
nanotubes made by the HiPCO method.
[0049] In some embodiments, the carbon nanotubes used in
conjunction with the methods and systems of the present disclosure
may include long carbon nanotubes. In more specific embodiments,
the long carbon nanotubes may have lengths that range from about 1
.mu.m to about 100 .mu.m, or from about 5 .mu.m to about 20
.mu.m.
[0050] Other suitable carbon nanotubes for use with various
embodiments of the present disclosure may include functionalized
carbon nanotubes. Such carbon nanotubes may be functionalized by
various functional groups, including but not limited to aryl
groups, alkyl groups, halogen groups, aromatic groups, carboxyl
groups, and the like. In more specific embodiments, the carbon
nanotubes may include SWNTs, such as pristine SWNTs.
[0051] Superacids
[0052] The methods and systems of the present disclosure may also
utilize various types of superacids. Superacids generally refer to
acids that have an acidity greater than that of 100% pure sulfuric
acid. Non-limiting examples of superacids suitable for use in
connection with the methods and systems of the present disclosure
include fuming sulfuric acid, oleum, chlorosulfonic acid, triflic
acid, fluorosulfonic acid, trifluoromethanesulfonic acid,
perchloric acid, anhydrous hydrogen fluoride, and combinations
thereof.
[0053] In some embodiments, superacids may also comprise Bronsted
acid/Lewis acid complexes. Such complexes can include, without
limitation, HSO.sub.3F/SbF.sub.5, HF/SbF.sub.5, HCl/AlCl.sub.3,
HF/BF.sub.3, and combinations thereof. In more specific
embodiments, the superacid used is chlorosulfonic acid (CSA). Other
suitable superacids may also be envisioned.
[0054] Without being bound by theory, it is envisioned that
superacids facilitate the dispersion or dissolution of CNTs by
surrounding the CNTs with a double layer of protons and
counterions. See, e.g., Davis et al., Macromolecules, 2004, 37,
154. It is likely that this proposed intercalation of ions is at
least partially responsible for the debundling of the CNTs before
immobilization onto a surface.
[0055] Surfaces
[0056] CNTs may be immobilized onto various surfaces (also referred
to as substrates) in accordance with the methods and systems of the
present disclosure. In some embodiments, the surfaces comprise
mesoporous and/or nonporous materials. Suitable surface materials
may include, without limitation, silicates, aluminosilicates,
silicon oxides, zeolites, glass, and quartz. In more specific
embodiments, the surface may comprise NaY zeolites (e.g.,
ultrastabilized NaY zeolites), USY zeolites (e.g., ultrastabilized
zeolite Y), and the like.
[0057] In some embodiments, the surface may comprise a polymeric
material. In some embodiments, the surface may include one or more
polymers. Suitable polymers on a surface may include, without
limitation, polyethylenes, polyethylene terephthalate,
polypropylenes, polystyrenes, polyethylene furanoate,
polycyclohexylenedimethylene terephthalate,
polytetrafluoroethylenes, fluorinated ethylene propylene,
perfluoroalkoxy, polyamides, polyimides, epoxies, aramids,
polyacrylonitriles, polyvinyl alcohols, polybutadienes, poly
acrylic acids, poly lactic acids, poly methacrylic acids,
polymethyl methacrylates, polyurethanes, poly vinyl chlorides,
polydimethyloxanes, polycarbonates, related materials, and
combinations thereof.
[0058] In some embodiments, the surface may comprise a Mobile
Composition of Matter surface (MCM), such as MCM-41. Specific
examples may include, without limitation, MCM-41-A and
MCM-41-S.
[0059] The surfaces of the present disclosure may also have various
forms, shapes, and structures. For instance, in some embodiments,
the surface may be a flat surface. In other embodiments, the
surface may have a circular shape. In further embodiments, the
surface may comprise glass beads and/or spheres (e.g., silicon
oxide spheres). In some embodiments, the surface may be patterned,
grooved, or otherwise non-planar. In yet further embodiments, the
surface can be patterned by ridges, grooves, or other patterns.
Other suitable surfaces can also be envisioned.
[0060] Methods
[0061] As set forth previously, various embodiments of the present
disclosure pertain to methods of fabricating carbon nanotube films.
Such methods generally comprise: (i) suspending carbon nanotubes in
a superacid to form a dispersed carbon nanotube-superacid solution,
wherein the carbon nanotubes have substantially exposed sidewalls
in the superacid solution; (ii) applying the dispersed carbon
nanotube-superacid solution onto a surface to form a carbon
nanotube film on the surface; and (iii) removing the superacid. The
aforementioned steps can occur under various conditions.
[0062] In some embodiments, the formed carbon nanotube-superacid
solution may be an isotropic solution. In some embodiments, the
isotropic solution comprises individually dispersed carbon
nanotubes. In some embodiments, the formed carbon
nanotube-superacid solution may be a biphasic solution. In some
embodiments, the biphasic solution comprises a mixture of both
individually dispersed carbon nanotubes and liquid crystals of
aligned carbon nanotubes. In some embodiments, the carbon nanotube
superacid solution is in liquid crystalline form. In some
embodiments, various polymeric or non-polymeric components may be
added to the carbon nanotube-superacid solution for optimal mixing.
Such components may also be added after mixing to create composite
films with interspersed segregated or multilayered structures.
[0063] In some preferred embodiments, the aforementioned steps of
the present disclosure occur without utilization of carbon nanotube
wrapping molecules or chemical functionalization. Such carbon
nanotube wrapping molecules may include surfactants (such as SDS),
oligonucleotides, and soluble silicon oxide molecules. In some
preferred embodiments, the methods of the present disclosure may
occur without utilization of any sonication steps. In some
embodiments, the methods of the present disclosure may occur
without the utilization of any carbon nanotube wrapping molecules,
chemical functionalization, or sonication steps.
[0064] The exclusion of carbon nanotube wrapping molecules and
sonication steps from the methods of the present disclosure provide
various advantages. For instance, the exclusion of such steps can
maintain the electronic properties of the immobilized carbon
nanotubes and make this method highly adaptable to long SWNTS.
Further, surfactant stabilization relies on sonication, which
shortens the CNTs. This also degrades film conductivity because it
raises the number of CNT-CNT junctions per unit area of the film.
Moreover, good surfactants adsorb strongly on CNTs. Thus, their
removal from the film is difficult. Furthermore, surfactant
residues in the final film increase sheet resistance
[0065] In some embodiments, other high performance materials that
are also soluble in acids may be added to the carbon
nanotube-superacid solution for molecular-level mixing. Such
materials may include, without limitations, graphenes, graphite,
fullerenes, boron nitride nanotubes, hexagonal boron nitride, and
combinations thereof. In related embodiments, a new class of
composite materials may be created by: (1) mixing and varying the
ratio of the added components in superacids, and/or (ii) performing
multiple coatings to create structured materials containing
multiple components.
[0066] Various methods may be used to apply dispersed carbon
nanotube-superacid solutions onto a surface. In some embodiments,
the applying step may include at least one of filtration, printing,
casting, coating, dip coating, die coating, rod coating, spray
coating, slot coating, gravure coating, slide coating, knife
coating, air knife coating, curtain coating, screen coating, and
combinations thereof. In some embodiments, the applying step
comprises dip coating. In some embodiments, the carbon nanotube
films fabricated using dip coating may have the carbon nanotubes
aligned in the dip coating direction.
[0067] Various methods may also be used to remove superacids from
formed carbon nanotube films. Such methods may include, without
limitation, direct coagulation, addition of polymers, evaporation,
filtration through a screen, and combinations thereof. In some
embodiments, the removing step is by direct coagulation, and the
step comprises dipping the carbon nanotube film in pure solvents or
mixtures of solvents. In some embodiments, the solvent comprises
chloroform, ether, isopropanol, acetone, ethanol, methanol, and
combinations thereof.
[0068] In some embodiments, the removing step may involve
evaporation. In more specific embodiments, the evaporation may
occur by various methods known in the art. Such methods may
include, without limitation, vacuum drying, freeze drying,
microwave drying and combinations thereof. In some embodiments, the
evaporation occurs by vacuum drying. In various embodiments, the
removing step may include, without limitation, heating by oven
drying, exposure to hot gas, microwaving, or combinations
thereof.
[0069] In some embodiments, the methods of the present disclosure
may also include a step of separating the formed carbon nanotube
film from a surface to produce a freestanding carbon nanotube film.
In some embodiments, such separation occurs by floating in water.
In other embodiments, the separation occurs by contact with another
surface. In yet other embodiments, the separation occurs by wetting
with another liquid that does not dissolve the carbon
nanotubes.
[0070] In some embodiments, the carbon nanotube films formed by the
methods of the present disclosure may be naturally acid-doped.
Acid-doping may be controlled by known methods. For instance, in
some embodiments, the removal step may be followed by washes in
diethyl ether and then water, or a wash in diethyl ether alone to
get the desired level of acid-doping. In some embodiments, the
doping can be performed with other molecules, such as Iodine.
[0071] Carbon Nanotube Films
[0072] The methods of the present disclosure may be utilized to
make various types of carbon nanotube films. As set forth
previously, carbon nanotube films of the present disclosure may
include a plurality of carbon nanotubes that are dispersed and
individualized. In some embodiments, the carbon nanotube films may
be freestanding. In some embodiments, the carbon nanotube films may
be immobilized onto a surface.
[0073] The carbon nanotubes on the carbon nanotube films of the
present disclosure may have various thicknesses. In some
embodiments, the carbon nanotubes on the films may have a thickness
that ranges from about 1 nm to about 1 .mu.m. In some embodiments,
the carbon nanotubes on the films may have a thickness that ranges
from about 10 nm to about 100 nm. In some embodiments, the carbon
nanotubes on the films may have a thickness that ranges from about
10 nm to about 20 nm.
[0074] The carbon nanotubes on the carbon nanotube films of the
present disclosure may also have various arrangements. In some
embodiments, the carbon nanotube films may include individualized
carbon nanotubes. In some embodiments, the individualized carbon
nanotubes may be isotropically oriented. In some embodiments, the
carbon nanotube films may include aligned carbon nanotubes. In some
embodiments, the carbon nanotube films of the present disclosure
may include bundles of aligned carbon nanotubes. In some
embodiments, the carbon nanotube films comprise isotropically
oriented bundles of aligned carbon nanotubes. In some embodiments,
the carbon nanotube films may comprise mixtures of isotropically
oriented carbon nanotubes and bundles of aligned carbon nanotubes.
In more specific embodiments, the liquid crystals of aligned carbon
nanotubes may have an ellipsoidal shape or a thread-like
appearance. In some embodiments, the carbon nanotube films of the
present disclosure may include mixtures of isotropically oriented
carbon nanotubes and bundles of aligned carbon nanotubes. In some
embodiments where the applying step is by flow coating (e.g., dip
coating, slot coating, roll coating, etc.), the bundles of the
carbon nanotubes are aligned in the coating direction.
[0075] In some embodiments, the carbon nanotube films may include
long carbon nanotubes. In more specific embodiments, the long
carbon nanotubes may have lengths that range from about 1 inn to
about 100 .mu.m, or from about 5 .mu.m to about 20 .mu.m.
[0076] The carbon nanotube films of the present disclosure may have
various advantageous properties. For instance, in some embodiments,
the carbon nanotube films of the present disclosure may have an
average transmittance of about 60% to about 100% at 550 nm. In some
embodiments, the carbon nanotube films may have an average
transmittance of about 10% to about 70%. In some embodiments, the
carbon nanotube films may have an average transmittance of about 1%
to about 40%. In some embodiments, the carbon nanotube films may
have an average transmittance below about 10%.
[0077] In some embodiments, the carbon nanotube films of the
present disclosure may have an average sheet resistance of about 20
ohm/sq to about 4,000 ohm/sq, from about 20 ohm/sq to about 1530
ohm/sq, from about 20 ohm/sq to about 200 ohm/sq, or from about 0.1
ohm/sq to about 50 ohm/sq. In some embodiments, the carbon nanotube
films of the present disclosure may have conductivities that range
from about 1.1.times.10.sup.5 S/m to about 3.1.times.10.sup.5 S/m,
from about 2.5.times.10.sup.5 S/m to about 5.5.times.10.sup.6 S/m,
from about 0.25.times.10.sup.5 S/m to about 1.5.times.10.sup.5 S/m,
from about 2.75.times.10.sup.5 S/m to about 8.5.times.10.sup.5 S/m,
or from about 7.3.times.10.sup.5 S/m to about 5.5.times.10.sup.6
S/m.
[0078] In sum, the carbon nanotube films made by the methods of the
present disclosure can be thin, conductive, and highly transparent.
As such, the methods of the present disclosure may be used to make
carbon nanotube films for various applications. For instance, in
some embodiments, the transparent, conductive, and thin films that
are made by the methods of the present disclosure may exceed those
of state-of-the-art indium tin oxide (ITO) films, while offering
competitive advantages such as flexibility, durability, resistance
to fatigue, and availability. Thus, in some embodiments, the films
made by the methods of the present disclosure may be used for
making touch screen panels and liquid crystal displays. In some
embodiment, the films made by the methods of the present disclosure
may be used in solar cells, flexible electronics, organic
light-emitting diodes, electromagnetic interference shielding, as
well as various antistatic, optical, and sensor coatings.
Additional Embodiments
[0079] From the above disclosure, a person of ordinary skill in the
art will recognize that the methods and systems of the present
disclosure can have numerous additional embodiments. Reference will
now be made to more specific embodiments of the present disclosure
and experimental results that provide support for such embodiments.
However, Applicants note that the disclosure below is for exemplary
purposes only and is not intended to limit the scope of the claimed
invention in any way.
EXAMPLES
[0080] Additional details about the experimental aspects of the
above-described studies are discussed in the subsections below.
Example 1
Materials and SWNT Solution Preparation
[0081] Carbon nanotubes used in the Examples herein are
single-walled carbon nanotubes (SWNTs) produced using the high
pressure carbon monoxide (HiPco) method (Rice University; batch
#188.2). The HiPco SWNTs have an average length of 500 nm and a
diameter of 1 nm. The amorphous carbon and iron catalyst were
removed by oxidizing at a temperature of 375 C in the presence of
SF.sub.6 and O.sub.2, followed by washing with 6M HCl at 90 C. Dip
coating solutions were prepared by stir-bar mixing for 24 hrs.
Volume fraction was calculated by assuming a density of 1300
kg/m.sub.3 for the SWNTs.
Example 2
Film Formation Process
[0082] The actual dip coating setup consists of a vertically
mounted syringe pump (Harvard Apparatus PHD 2000) as shown in the
schematic diagram (FIG. 1B). Different withdrawal speeds were
obtained by controlling the volumetric flow rates for an
arbitrarily assigned syringe diameter. The dip-coated film contains
both SWNT and the solvent (i.e. chlorosulfonic acid). As soon as
the liquid film was withdrawn from the dip coating solution, it was
immersed in chloroform for at least 15 mins to remove the acid. It
should be noted that upon moisture exposure, chlorosulfonic acid
decomposes into hydrochloric gas and sulfuric acid, causing
significant de wetting of the film, due to high surface tension of
sulfuric acid. To avoid moisture contamination, the whole operation
was carried out inside a glove bag constantly purged with anhydrous
argon gas, as shown in FIG. 1B.
Example 3
Characterizations
[0083] Light microscopy images of the dip coating solutions and
final SWNTs films were captured using a Zeiss Axioplan optical
microscope. Liquid samples were prepared inside an anhydrous glove
box as described, where the sample solution was confined between a
microscope slide and cover slip, and sealed with aluminum tape. SEM
images were captured using JEOL 6500F. For TEM samples, the SWNT
film was detached from the glass substrate by submerging the sample
into DI water. A TEM grid was then used to recover the detached
part of the film for imaging (JEOL 2010, 100 kV). Steady shear
rheology of the solutions was characterized using a
stress-controlled rheometer (AR 2000ex; TA Instruments) enclosed
inside a custom-made glove box constantly purged with dry air.
Relative humidity during sample loading and testing was kept below
2-3%. Parallel plate geometry made of stainless steel (SS 316) was
used. SWNT film thickness (after chloroform quenching) was measured
using mechanical profilometer (Vecco Dektak 6M) and atomic force
microscopy (AFM; Digital Instruments Nanoscope IIIA). For
profilometer measurements, the error bar of each data point in FIG.
6 represents the standard deviation of nine measurements (three
films were dip coated from the same solution and three measurements
were performed at various locations on each film). For AFM
measurements, the error bar represents three measurements carried
out at different locations of the same film. Films produced from
the 0.5% solution were measured using both AFM and profilometer,
and the measurements agree to within 10%. Sheet resistance was
measured using the 4-point probe method (Jandel), and transmittance
at 550 nm was measured by UV-Vis spectrometer (Shimadzu
UV-1800).
Example 4
CNT Solution Preparation and Set-Up
[0084] Purified DWNTs were purchased from CCNI (batch X647H). HiPco
SWNTs (batch 188.3) were produced at Rice University and purified
according to literature methods. The average length of DWNTs was
estimated to be about 10 .mu.m and they were mostly few-walled
nanotubes (single, double and triple walls with an average wall
number of 2.25, and an external diameter of about 2.4 nm). CSA was
used as received (grade 99%, purchased from Sigma-Aldrich). The
CNTs and CSA were initially mixed at .about.10000 ppm in a
speedmixer (DAC 150.1 FV-K, Flack Tek inc). This stock solution was
then diluted to the coating concentrations by further speedmixing
for 10 to 20 minutes, followed by stir-bar mixing for 24 hours.
Each film was coated on a glass slide previously cleaned with
acetone (C.sub.3H.sub.6O) and then air dried. A motorized stage
(vertically-mounted syringe pump, Harvard Apparatus PHD 2000) was
used to immerse and lift the glass slide into and out of both the
CNT-CSA solution and chloroform bath at prescribed speed. In the
case of simple chloroform coagulation (method (1)), the films was
immersed in chloroform for at least 20 minutes to ensure complete
removal of the CSA. Finally, the film was annealed at 115.degree.
C. for 30 minutes to improve their adhesion to the glass support.
When the complete sulfuric acid removal was desired (method (2)),
the glass slide was first immersed in chloroform for 20 minutes
after fabrication. Then, the glass slide was left in a diethyl
ether bath for 3 minutes. After the ether washing, the film was
annealed in the oven at 115.degree. C. for 15 minutes to improve
the adhesion to the glass slide. Finally, the glass slide was
immersed in a water bath to remove the residual sulfuric acid. The
process was completed with another 15 minutes in the oven at
115.degree. C. to dry the film. The intermediate film annealing
between diethyl ether and water wash is necessary to avoid the
detachment of the film from the substrate. The CSA removal was also
performed using direct CSA evaporation (method (3)) as described
elsewhere. Saha, A.; Ghosh, S.; Weisman, R. B.; Marti, A. A., Films
of Bare Single-Walled Carbon Nanotubes from Superacids with
Tailored Electronic and Photoluminescence Properties. ACS Nano
2012, 6(6) 5727-5734. In brief, the film was heated in a vacuum
oven at 150.degree. C. after fabrication for 20 minutes. Then, the
glass slide was immersed in a diethyl ether bath for 3 minutes and
dried in oven for another 10 minutes. The whole dip-coating and
coagulation process was performed in a glove box purged
continuously with dry air in order to keep the moisture
concentration less than 10%. The presence of water vapor could
result in an exothermic reaction between the residual moisture and
chlorosulfonic acid that may affect the integrity of the films due
to the generation of HCl gas. The film deposited on one side of the
glass slide was wiped off before the transmittance
measurements.
Example 5
Characterization
[0085] The CNT film morphology was studied using a Zeiss Axioplan
optical microscope. TEM images were captured using JEOL 2010. The
TEM sample preparation was achieved by immersing the dip-coated
slides into a DI water bath after chloroform coagulation and
transferring of the floating films onto a TEM grid. Films produced
in this manner can be easily detached from the glass slide and
transferred to other substrates. FEI quanta 400ESEM FEG was used to
obtain the SEM pictures. The transmittance of the films at the
wavelength of 550 nm was measured by a UV-Vis spectrometer
(Shimadzu UV-1800), while the sheet resistance was obtained with a
linear four-point probe device (Jandel model RM3-AR). The XPS
spectra and the rheology data were obtained using PHI Quantera XPS
and AR2000eX (TA Instruments) with a concentric cylinder Couette
geometry, respectively.
Example 6
Formation of Randomly Oriented SWNT Films from Isotropic
Solutions
[0086] Single-walled CNT (SWNT) films were formed in a three-step
process: formation of a superacid solution; coating the solution on
a glass substrate; consolidation of structure by solvent removal.
First, SWNTs were individually dispersed in chlorosulfonic acid at
low concentrations (0.1% and 0.2%), forming single-phase isotropic
solutions. These solutions were then coated onto a glass substrate
by dip coating. Because of the low volatility of chlorosulfonic
acid, the acid was subsequently removed from the wet film by
immersion in chloroform. This solvent extraction step resembles the
coagulation step in wet spinning of neat CNT fibers and differs
from conventional dip coating, where the (volatile) solvent is
evaporated away. The resulting solid films have a thickness of tens
of nm (FIGS. 1A and 1B; Table 1), show no optical birefringence,
and consist of randomly oriented SWNTs (FIG. 1c). They have an
average transmittance of 67% and 90% (at 550 nm), and an average
sheet resistance of 135 and 1530 .OMEGA./sq respectively (Table 1),
potentially useful as flexible, transparent, and conductive
coatings for touch screen applications.
TABLE-US-00001 TABLE 1 Sheet resistance and final film thickness of
SWNT films prepared from different concentrations of SWNT-superacid
solutions (C.sub.SWNT). Each sheet resistance value was calculated
based on measuring three different locations of three films dip
coated from the same concentration solution. Film thickness for the
0.1% and 0.2% films was measured by AFM, whereas that for the 0.5%,
0.7%, and 1% films was measured by a mechanical profilometer.
h.sub.dry C.sub.SWNT R [.OMEGA./sq] [nm] 0.1%* 1531 .+-. 313 10
.+-. 6 0.2%* 135 .+-. 36 23 .+-. 4 0.5% 62 .+-. 35 143 .+-. 16 0.7%
22 .+-. 1 201 .+-. 35 1% 8 .+-. 3 397 .+-. 51 *Note: SWNT whiskers
were NOT observed in films dip coated from the isotropic solutions
(0.1% and 0.2%).
Example 7
Formation of Whisker-Like SWNT Crystallites from Biphasic
Solutions
[0087] To attain films containing highly aligned, whisker-like SWNT
crystallites, dubbed hereafter `SWNT whiskers`, a higher
concentration solution was used (FIG. 2). As the CNT concentration
grows, excluded volume interactions cause the formation of liquid
crystalline (LC) domains consisting of aligned CNTs; the solution
transitions from an isotropic phase of randomly oriented CNTs to a
biphasic system, where a liquid crystalline phase co-exists with an
isotopic phase. In this regime, concentration and acid strength
control the morphology of the LC domains, which can vary from
nearly endless threads (spaghetti) to ellipsoidal (acicular) as the
solvent goes from sulfuric to chlorosulfonic acid. At even higher
concentrations, CNT in acids form a LC with polydomain morphology.
FIG. 3 shows the optical microstructure of a biphasic
solution--0.7% (vol.) SWNT in chlorosulfonic acid--captured using
transmitted light, with (FIG. 3B) and without (FIG. 3A)
cross-polarizers. As shown in the polarized light micrograph (FIG.
3B), several discrete LC structures in the solution show optical
birefringence ("lit up"), indicating intrinsic alignment of SWNT
within these structures. These LC structures have a peculiar
ellipsoidal (acicular) shape, likely due to the strong elastic
energy (splay and bend) and low interfacial tension in the system.
Similar acicular shape LC structures were recently reported for
aqueous dispersions of bile-salt stabilized SWNTs, but the director
within these structures were found to be uniform rather than
bipolar. Unlike the convective assembly method reported for aqueous
CNT solutions where the gradual evaporation needed to concentrate
the solution and form a liquid crystal occurs over several days,
the acid removal process generally takes minutes or less to
complete and therefore has a much shorter processing time.
[0088] FIGS. 3C and 3D show representative light micrographs of the
resulting film captured using transmitted light (with and without
cross-polarizers). As shown in the figures, the resulting film
contains structures that are preferentially aligned along the dip
coating direction. No preferential alignment of LC domains is
observed in the biphasic solutions. Therefore, the streamwise
alignment must be caused by the dip coating process. Because the LC
domains are fluid, they may undergo stretching in addition to
alignment in the high shear film formation zone. These elongated LC
domains then act as nucleating sites for the further growth of
large, SWNT whiskers during subsequent acid removal process (FIG.
2). SWNT whiskers are typically 10 to 20 .mu.m long and .about.100
nm thick. They are strongly birefringent in polarized light
microscopy (FIG. 3D), indicating that the SWNTs within these
structures are highly aligned. It is worth noting that in the case
of thermotropic liquid crystalline polymers, macroscopic alignment
of polymer crystallites can be obtained by applying shear to the
specimen during annealing. In the present disclosure, the inventors
show that by using the right solvent and processing flow
conditions, SWNTs can be processed in an analogous way to obtain
crystallite structures with macroscopic alignment. Transmission and
scanning electron microscopy (TEM and SEM) were performed on the
dried films. Interestingly, SWNT whiskers are only visible in
transmission mode (TEM) (FIGS. 4B& 4C) but not in surface
scanning mode (SEM) (FIG. 4A), indicating that these structures are
embedded within a random network of SWNT. This is similar to the
case of crystallites found in polymer melt systems, where
crystallites cannot be captured by SEM unless the amorphous portion
of the specimen is etched selectively. TEM shows that SWNT whiskers
typically have a maximum width of 100 nm. Their alignment may not
be fully resolved because they are embedded in a random SWNT
network. Nevertheless, as part of the film was broken and
transferred from the glass substrate onto a TEM grid, some SWNT
whiskers protruded out of the random network at the edge of the
specimen (FIG. 4C). Clear alignment of SWNT within the whisker is
shown in FIG. 4D. FIG. 4E shows the corresponding electron
diffraction pattern containing information about SWNTs packing
within the whisker. Henrard et al. showed in their electron
diffraction simulations and experiments that clear peaks along the
central line are present in SWNT bundles, due to the tubes being
packed in a triangular lattice. The exact position and breadth of
these peaks are influenced by average diameter, diameter
distribution and overall alignment. Two peaks were observed at 0.9
.ANG..sub.-1 and 1.4 .ANG..sub.-1 along the central line of the
electron diffraction of the SWNT whisker (FIG. 4F), in qualitative
accord with the published patterns. This indicates that SWNTs are
aligned as well as packed in a crystalline manner (inset of FIG.
4F), and may be of larger diameter than 16.8 .ANG. analyzed by
Henrard et al. The first and second layer lines are neither
uniformly intense, nor clearly spotty. Such feature is typical of
bundles with mixed chirality indicating that SWNT whiskers are made
of SWNTs with mixed chirality.
Example 8
Macroscopic Alignment of SWNT Whiskers
[0089] To understand why SWNT whiskers are aligned but individual
SWNTs are not, two dimensional steady state flow equations were
solved numerically by using a previously published Galerkin-finite
element methods. The input parameters for the modeling were
obtained by fitting the rheological data to a power-law fluid
model. The surface tension of the chlorosulfonic acid solutions was
measured to be 22.31 mN/m using the pendant drop method (KSV CAM
200 Contact Angle Tool). FIGS. 5A-5D show the shear rate profile
generated during the dip coating process for different
concentrations of SWNT solutions. In the dip coating process, the
highest local shear rate occurs close to the moving substrate and
decreases to zero as the free surface is approached. The average
shear rate is defined as:
.gamma. . _ s = .intg. A e .gamma. . s A .intg. A e A = .intg. A e
.gamma. . s A A e ##EQU00001##
where A.sub.e is the liquid entrainment area, and it contains
elements with a stream function value higher than that at the
separating streamline (see FIG. 5E). For SWNTs and LC structures to
align, the Peclet number should be large, i.e.,
Pe = 1 , D r 1 , ##EQU00002##
where D.sub.r is the rotary diffusivity. The rotary diffusivity of
individual SWNTs for the semi-dilute, isotropic 0.1% and 0.2%
solutions is estimated to be 0.8 s.sub.-1 and 0.2 s.sub.-1,
respectively, whereas that for LC structures is estimated to be
.about.0.001 s.sup.-1 54. The highest local shear rate generated
from dip coating biphasic solutions ranges from 11-36 s.sup.-1.
Therefore, the Pe number for LC structures is of the order of
10,000 (indicating strong flow alignment) while it is several
orders of magnitude smaller (.about.10 to 50) for individual SWNTs
(indicating weak alignment). Moreover, the rotational relaxation
time
.tau. r = 1 6 D r ##EQU00003##
of the whiskers is in the range of tens of minutes, indicating that
the whiskers cannot reorient before solvent extraction, whereas the
SWNT can rearrange in less than one second and would therefore lose
any flow alignment as the liquid film is transferred into
chloroform. Discrete LC structures deform when the hydrodynamic
forces are strong enough to overcome the elastic and interfacial
ones. The capillary number Ca for a LC structure suspended within
an "isotropic SWNT" medium is estimated to be .about.4, i.e., the
flow-induced shear stresses on the interface LC tactoids exceed the
interfacial tension. Similarly, the Ericksen number Er is estimated
to be on the order of 10, i.e. the shear stresses are sufficiently
high to overcome the elastic stresses due to deformation of the
domains. This simple scaling analysis suggests that the discrete LC
structures are stretched in the high shear flow during film
formation. The orientation distribution of whiskers and the angle
of misalignment from the dip coating direction were calculated
using ImageJ with plugin "OrientationJ". As shown in Table 2,
whiskers in 1% film have a larger angle of misalignment
(.+-.14.1.degree.) from the dip coating direction compared with
those in the 0.5% and 0.7% films (.+-.10.4.degree.). This can be
explained by considering the shear rate and strain experienced by
LC structures. As the solution concentration increases, both the
maximum and average shear rate {dot over ( .gamma..sub.s decrease.
The residence time t.sub.R is:
t R = .intg. A e A q = A e q , ##EQU00004##
where q is the flow rate (2-D) through the outflow boundary.
Because the flow rate q grows faster than the liquid entrainment
area A.sub.e, both the residence time t.sub.R and the total strain
(=t.sub.R) experienced by the SWNT decrease. The lower alignment in
the 1% film can therefore be explained by: (1) lower average shear
rate and (2) smaller strain experienced by the LC structures during
the dip coating process. Table 2-Degree of whisker misalignment (
{square root over (.theta..sup.2)}) for films dip coated from
different concentrations of SWNT-acid solutions (0.1%, 0.2%, 0.5%,
0.7%, and 1%), where .theta. is the angle between the whisker axis
and the dip coating direction. The orientation distribution of
whiskers was calculated from the optical micrographs using ImageJ
plugin OrientationJ, and each reported {square root over
(.theta..sup.2)} value was averaged over 3 arbitrarily chosen areas
(300 .mu.m.times.300 .mu.m).
TABLE-US-00002 EXAMPLE 9 Films dip coated from {square root over (
.theta..sup.2 )} Isotropic 0.1% No whisker formation solutions 0.2%
No whisker formation Biphasic 0.5% .+-.10.4.degree. solutions 0.7%
.+-.10.4.degree. 1% .+-.14.1.degree.
Film Thickness and Sheet Resistance
[0090] Film thickness depends on SWNT concentration and substrate
withdrawal speed. For a fixed substrate withdrawal speed, lower
concentration solutions produce thinner films because they have
lower viscosities and contain fewer SWNTs, as shown in FIG. 6A and
Table 1. FIG. 6B shows film thickness (after acid removal) as a
function of substrate withdrawal speed. Gutfinger and Tallmadge
applied lubrication analysis to power-law fluids
(.eta..sub.a=K.sup.-1) and showed that the film thickness should
scale as:
h dry .varies. h wet .varies. U o 2 n 2 n + 1 , ##EQU00005##
where U.sub.o is the substrate withdrawal speed. Table 3 compares
the predicted scaling exponents (using n values from rheological
measurements) with those determined from FIG. 6B. The values for
0.5% and 0.1% solutions agree to within .about.2% and those for the
0.7% solution agree to within .about.20%. The sheet resistance of
the SWNT films is reported in Table 1. Films dip coated from higher
concentration of SWNT are thicker and have lower sheet resistance.
Based on the sheet resistance and thickness data, the conductivity
for the 0.5%, 0.7% and 1% films containing SWNT whiskers varies
from 1.1.times.10.sup.5 S/m to 3.1.times.10.sup.5 S/m, which is
comparable to values reported for SWNT films produced from
filtration (using HiPco nanotubes) and acid doped SWNT
fibers.sub.11 (5.times.10 S/m). Although most of the acid has been
removed during the coagulation step, the films are naturally doped
with trace amounts of adsorbed acid, which increases their
conductivity. Acid doping can be removed by heat treatment,
yielding .about.three-fold higher sheet resistance after heating at
150.degree. C. for 24 hours.
TABLE-US-00003 TABLE 3 Scaling exponent (.alpha.) for film
thickness (h) as a function of substrate withdrawal speed (U.sub.o)
(i.e. h.sub.dry .varies. U.sub.o .sup..alpha.). Comparison between
.alpha. obtained from lubrication analysis .sup.58 and from FIG.
6b. .alpha. predicted in n value from lubrication analysis (i.e.
.alpha. from C.sub.SWNT rheology 2n/(2n + 1)) FIG. 6b 0.5% 0.47
0.49 0.48 0.7% 0.45 0.48 0.37 1.0% 0.36 0.42 0.41
Example 10
[0091] Thin films were fabricated starting from solutions of CNTs
in CSA. Both HiPco SWNTs (length L.about.0.0.5 .mu.m, diameter
D.about.1 nm) and DWNTs (L.about.10 .mu.m, D.about.2.4 nm) were
used. SWNTs and DWNTs were dissolved (without sonication) in CSA at
1000, 2000, and 3000 ppm wt % (deposition from lower concentration
solutions yielded sparse CNT coverage, high transparency
.about.99.5%, and high sheet resistance R.sub.s.about.12
k.OMEGA./sq). Beyond a critical concentration w.sub.1, CNTs form
biphasic solutions with an isotropic (randomly oriented) phase in
equilibrium with a nematic liquid crystalline phase. This optimal
concentration scales inversely with CNT aspect ratio as
w.sub.1.about.D/L. The measured transition concentrations were 4100
ppm for SWNTs and 125 ppm for DWNTs. Therefore, the SWNT solutions
were isotropic, whereas the DWNT solutions contained a small amount
of nematic phase (.about.10 to 20% depending on overall
concentration--see FIG. 13 for images of SWNT and DWNT solutions).
Glass slides were lowered into the CNT-CSA solution and were
withdrawn at a controlled speed by a motorized arm (see FIG. 7).
Three methods were used to remove CSA from the films: (1)
coagulation by immersion of the glass slide in chloroform
(CHCl.sub.3) followed by oven drying; (2) coagulation by immersion
of the glass slide in chloroform, followed by washes in diethyl
ether (C.sub.4H.sub.10O) and then water, with final oven drying;
(3) direct evaporation of CSA in a vacuum oven at 150.degree. C.
followed by diethyl ether wash and drying as shown in previous
literature. FIG. 7E shows an example of a 90% transmittance film
fabricated using the dip coating technique. All three methods
yielded homogeneous films (FIG. 8A) but different amounts of
residual sulfuric acid (H.sub.2SO.sub.4) doping (see discussion
below). Chloroform was chosen as the coagulant because it dissolves
CSA without reacting (unlike water that forms hydrochloric acid
(HCl) gas and sulfuric acid), which can damage the film structure.
Because of its high volatility, chloroform rapidly evaporates from
the film once the slide is removed from the bath. However,
chloroform is not a good solvent for sulfuric acid and hence leaves
residual acid in the film. Thus, washing in diethyl ether and water
may be desired to remove sulfuric acid whenever sulfuric acid
doping is not desired (as discussed below). The use of these three
methods allowed the study of film electrical properties depending
on the residual acid level.
Example 11
[0092] SWNT and DWNT films displayed distinguishable morphologies
(FIG. 8). Under cross polarized light (FIG. 8B), no ordered
structure was observed in SWNT films coated at low speed (1-2
mm/min), whereas small birefringent regions are observable in films
coated at higher speed (3 mm/min) Conversely, all DWNT films showed
elongated birefringent domains aligned along the coating direction
at all concentrations. This morphology is consistent with the
microstructure of the coating solutions and the action of the shear
field. In isotropic SWNT solutions, at low speed, the shear rate
was insufficient to produce ordering in the films, whereas some
shear-induced ordering was observed at high speed. Conversely, the
pre-existing liquid crystalline domains in the DWNT solutions were
stretched and aligned by the shear field.
Example 12
[0093] Due to the CNT orientation, the DWNT films were expected to
display anisotropic electrical properties. Sheet resistance was
measured with a linear four-point probe at three different angles
with respect to the coating direction and found no angular
variation irrespective of the CSA removal technique (for example, a
typical method (1) film had 117.0.+-.12.6 .OMEGA./sq, 117.8.+-.11.4
.OMEGA./sq, and 118.3.+-.8.0 .OMEGA./sq at 0.degree., 45.degree.,
and 90.degree., respectively at .about.85% transmittance). Further
study of the film morphology (FIG. 9) using Scanning Electron
Microscope (SEM) and Transmission Electron Microscope (TEM) showed
large bundles aligned along the coating direction (responsible for
the optical birefringence) connected through a network of thinner
bundles and individual CNTs predominantly aligned perpendicular to
the large bundles. These perpendicular structures ensure isotropic
film conductivity. They may arise from the isotropic phase present
in the solution or be induced by vorticity aligning in the shear
flow (known to occur in liquid crystalline polymers and CNT
fluids).
Example 13
[0094] The thickness, h.sub.wet, of the dip coated liquid film,
called the wet film thickness, is controlled by the interplay of
surface tension and gravity, which oppose film formation, and
viscous forces, which draw liquid from the coating bath onto the
substrate. Whereas surface tension and gravity are
process-independent, viscous forces can be controlled by the
withdrawal speed u and solution CNT concentration (which affects
viscosity). CNT concentration also affects the dry film thickness
through h.sub.wet=h.sub.dry .phi., where .phi. is the CNT volume
fraction in the coating liquid. FIG. 4 shows how transmittance and
sheet resistance (both related to the dry film thickness) change
with withdrawal speed at different DWNT concentrations. As
expected, higher withdrawal speed and higher CNT concentration
yield thicker films (lower transmittance and lower sheet
resistance). For Newtonian fluids, the relationship of wet film
thickness to process parameters (the well-known Landau-Levich
relation) is a function of the capillary number (ratio of viscous
to surface tension forces), or, in terms of velocity,
h.sub.wet.about.u.sup.2/3. However, CNT solutions in CSA are
non-Newtonian. In the range of measurements, they shear thin as
power-law fluids with apparent viscosity .eta..sub.a=K{dot over
(.gamma.)}.sup.n-1, where {dot over (.gamma.)} is the shear rate, n
is the power law exponent (n=1 is a Newtonian fluid), and K is the
consistency index (see rheology measurements in FIG. 14). Gutfinger
and Tallmadge extended the Landau-Levich model to power-law fluids,
obtaining
h wet .varies. u 2 n 2 n + 1 . ##EQU00006##
Applicants' film thickness data (FIG. 15) is closer to the
Newtonian behavior, probably due to the low shear rate at which the
films are produced (shear thinning may influence the films produced
at higher velocity).
Example 14
[0095] FIG. 10B shows sheet resistance versus transmittance. As
expected, thicker films (lower transmittance) exhibit better
electrical properties due to the increased number of pathways that
the electrons can travel through. All the data fall on the same
process curve spanning .about.2 orders of magnitude in sheet
resistance, indicating that this simple coating process is very
robust and versatile. Interestingly, film electrical properties
depend on the fabrication technique used: the best electrical
properties were obtained with films made by direct CSA evaporation
and diethyl ether wash (method (3)), followed by chloroform
coagulated films (method (1)) and water washed films (method (2)).
The difference in electrical properties using the three techniques
is related to the presence of sulfuric acid (H.sub.2SO.sub.4) that
can be completely or partially removed from the film depending on
the washing procedure. CSA reacts with the moisture in the air,
leading to the formation of sulfuric acid that acts as a doping
agent. Complete sulfuric acid removal was achieved by washing the
films in water (method (2)), while some residual sulfuric acid
appeared in films produced using method (1) and (3) (<0.1 atomic
% for method (2), while method (1) and (3) showed .about.9.1 and
2.3 atomic % in sulfur content, respectively--see X-ray
photoelectron spectroscopy (XPS) data in FIG. 16). Despite the
higher sulfuric acid content, method (3) yielded films with time
stability comparable to method (2) (less than 14% variation in 9
days), whereas films fabricated by method (1) showed increasing
sheet resistance with time (see FIG. 17). Such results are
unexpected because films made by method (3) contain sulfur and are
about twice more conductive than films made by method (2).
Interestingly, films made by method (1) and (3) showed a rapid
increase in sheet resistance (.about.40 and 21%, respectively) when
a constant current of 1 mA was applied. This increase was
reversible and is likely related to ionic conductivity due to
residual sulfuric acid in the film (see FIG. 18).
Example 15
[0096] Films fabricated from different CNTs are expected to exhibit
different performance due to the respective quality, length, and
diameter of the constituent CNTs. Although long CNTs are more
difficult to disperse in liquids, they are desirable because the
film conductivity scales as L.sup.1.46, where L is the average CNT
length. When using CSA, length is not a barrier to dissolution.
Comparing films made of SWNT and DWNT; within the same fabrication
method, SWNT films had .about.4 to .about.10 times higher sheet
resistance than corresponding DWNT films (FIG. 11), demonstrating
the importance of the length and quality of CNTs for producing
high-performance films; for example (method (1)), at .about.88%
transmittance, SWNT and DWNT films have a sheet resistance of
.about.1300 .OMEGA./sq and 140 .OMEGA./sq, respectively, while at
.about.97% transmittance, SWNT and DWNT films have a sheet
resistance of .about.3200 .OMEGA./sq and 850 .OMEGA./sq.
Example 16
[0097] Compared to CNT films in the dip-coating literature (FIG.
12A), the films from CSA-CNT solutions show optimal properties,
likely due to the CNT length and quality as well as film
morphology. FIG. 6B compares findings of the present disclosure
using method (2) and (3) with the best values published in
literature to date using various techniques. The best performance
was obtained by Hecht et al., who made DWNT films by filtration
from CSA solutions, but reported short-term stability issues. The
.about.2.times. performance difference may be due to the absence of
liquid crystalline domains in their low-concentration (.about.100
ppm) solutions, indicating that further improvements may be
attained by moving from dip coating to pre-metered coating methods
(which can operate at lower solid concentration and lower
viscosity). Without any chemical post-treatment, the films show
similar properties to those obtained by post-treatments with either
nitric acid (HNO.sub.3) or thionyl chloride (SOCl.sub.2), known to
dramatically improve the film properties by doping but also give
poor electrical stability in time.
[0098] Without further elaboration, it is believed that one skilled
in the art can, using the description herein, utilize the present
invention to its fullest extent. The embodiments described herein
are to be construed as illustrative and not as constraining the
remainder of the disclosure in any way whatsoever. While the
preferred embodiments have been shown and described, many
variations and modifications thereof can be made by one skilled in
the art without departing from the spirit and teachings of the
invention. Accordingly, the scope of protection is not limited by
the description set out above, but is only limited by the claims,
including all equivalents of the subject matter of the claims. The
disclosures of all patents, patent applications and publications
cited herein are hereby incorporated herein by reference, to the
extent that they provide procedural or other details consistent
with and supplementary to those set forth herein.
* * * * *